Filamentary electrode and fabrication thereof

ABSTRACT

A filamentary electrode of the coiled coil loose fitting overwind type for low pressure discharge lamps. The fine overwind is coiled in oversized primary turns around the permanent primary mandrel wire which in turn is coiled into non-circular, suitably oval or egg-shaped secondary turns. The overwind is thereby prevented from unscrewing from the primary mandrel wire and the need for crimping, flattening or welding the ends of the coil prior to dissolving out the temporary mandrel wires used in coiling is avoided. Where there is further coiling into tertiary turns as in triple-coiled electrodes, tangling and shrinkage are minimized.

United States Patent [1 1 Miller, Jr. et al.

[.11 3,736,458 1 May 29,1973

FILAMENTARY ELECTRODE AND FABRICATION THEREOF [75] Inventors: Victor W. Miller, Jr., Highland Heights; John L. Ballard, Columbus, both of Ohio [73] Assignee: General Electric Company, Schenectady, N.Y.

[22] Filed: July 6, 1971 [21] Appl. No.2 159,801

52 us. Cl ..313/344 [51] Int. Cl. ..H0lk 1/14 [58] Field of Search ..3l3/27l, 344

[56] References Cited UNITED STATES PATENTS 2,394,474 2/1946 Peters .313/344 3,253,894

5/1966 Heine.....

3,294,125 12/1966 Heine ..3l3/344X Primary Examiner-David Schonberg Assistant zsqminqrrPanl -Sesher Attor riy-Emest W. Legree, Henry P. Truesdell, Frank L. Neuhauser et al.

[57] ABSTRACT A filamentary electrode of the coiled coil loose fitting overwind type for low pressure discharge lamps. The fine overwind is coiled in oversized primary turns around the permanent primary mandrel wire which in turn is coiled into non-circular, suitably oval or eggshaped secondary turns. The overwind is thereby prevented from unscrewing from the primary mandrel wire and the need for crimping, flattening or welding the ends of the coil prior to dissolving out the temporary mandrel wires used in coiling is avoided. Where there is further coiling into tertiary turns asin triplecoiled electrodes, tangling and shrinkage are minimized.

6 Claims, 5 Drawing Figures Patented May 29, 1973 T L Tm o o .TWB .T rL A mmn .m .mmw m v T FILAMENTARY ELECTRODE AND FABRICATION THEREOF BACKGROUND OF THE INVENTION The invention relates to electrodes for low pressure electric discharge lamps such as fluorescent lamps and to their construction and fabrication.

The requirements for cathodes of fluorescent lamps are somewhat contradictory. On the one'hand, it is desirable that the cathode should heat up and become fully emissive very quickly which suggests a construction consisting of fine refractory metal wire compactly coiled and coated with electron emissive material. On the other hand, it is desirable that the active cathode surface should all be at potentials within a restricted range during operation which suggests a more massive structure to afford high conductivity. Also the need to avoid local overheating as at the spot where the electric discharge or are strikes suggests a more massive structure in order to achieve higher thermal conductivity. The art has found the answer to these conflicting requirements in the so-called overwound cathode which comprises a fine wire wound loosely around a heavier core or mandrel wire with the resultant composite wire wound into the form of a coil. Such a structure is generally referred to as a double-coiled overwound cathode. When this coil is in turn wound into a still larger coil, it is referred to as a triple-coiled overwound cathode. In each case it is only the overwind which is either double-coiled or triple-coiled as the case maybe, and the heavier core or mandrel wire is correspondingly merely single-coiled or double-coiled. In order to achieve a larger carrying capacity for electron emissive material, the overwind is formed to fit loosely on the mandrel wire. This is achieved by pairing a temporary filler wire with the core or mandrel wire before applying the fine overwind wire. The common practice is to use tungsten for the permanent wires, that is for the mandrel wire and the overwind, and to use molybdenum for the temporary filler wire, and this permits the filler wire to be chemically dissolved by acid and removed from the coil structure after winding.

While the overwound cathode is excellent as regards its operating characteristics, it presents manufacturing difflculties which increase its cost. Because the fine overwound wire is held only loosely by the primary mandrel wire, the overwind tends to slip back from the end of the coil leg and expose the primary mandrel wire. Also the overwind may slip off the end of the'filament leg and form a burr. In either case tangling is promoted and shrinkage results because the fine overwind wire fails to extend into the mount clamps for securing the filamentary electrode to the support wires of the lamp stem.

SUMMARY OF THE INVENTION The object of the invention is to provide a filamentary electrode of the loose fitting overwind type wherein the overwind is retained in place as an inherent feature of the construction. With such a feature, the tendency of the overwind to slip'back from the end of the coil leg and expose the primary mandrel is eliminated as is also the tendency of the overwind to slip off the end of the filament leg and form a burr. It is desired to provide a construction which is easy to manufacture and wherein shrinkage is minimized and quality of the product is enhanced.

In accordance with our invention, the fine overwind is coiled in loose fitting oversized primary turns around the permanent primary mandrel wire which in turn is coiled into non-circular secondary turns. The turns are most conveniently oval or egg-shaped. After heat setting, the eccentricity in the turns of the mandrel wire causes the overwind to be retained in place and preventsit from unscrewing from the primary mandrel wire so that it can neither slip back nor forward.

A preferred method for making a double-coiled overwound or so-called stick cathode by way of example is as follows. A fine tungsten overwind wire is wrapped around two parallel primary mandrels, one of tungsten to form the permanent mandrel or core wire, and the other of molybdenum to form a temporary mandrel which will eventually be chemically removed. The assembly consisting of the two primary mandrel wires and the overwind wire is now coiled around two parallel secondary mandrel wires which are likewise temporary and suitably ofmolybdenum. The secondary mandrel wires may be either equal or unequal in diameter resulting in the turns being either oval shaped or egg shaped, respectively. In either case the eccentricity. in the secondary turns which is made permanent by heat setting preventsthe overwind wire from unscrewing from the primary tungsten mandrel or core wire which remains after the tungsten wires have been chemically dissolved and removed.

DESCRIPTION OF DRAWINGS In the drawing:

FIG. 1 is a pictorial view of a fluorescent lamp stem whereon is mounted a coiled-coil or stick cathode embodying the invention.

FIG. 2 is a cross-sectioned end view through the primary mandrels showing a single turn of the overwind.

FIG. 3 is a fragmentary side view of the secondary mandrels with two turns of the primary mandrelsoverwind composite wrapped around it.

FIG. 4 is a cross-sectional end view through the secondary mandrels of FIG. 3.

FIG. 5 is a fragmentary side view of a few turns of the coiled-coil electrode after the forming mandrels have been dissolved.

DESCRIPTION OF PREFERRED EMBODIMENT Referring to the drawing, FIG. 1 illustrates a coiled coil or so-called stick cathode mounted on a lamp stem 1 suitable for a fluorescent lamp of the rapid start or switch start type. The electrode mount comprises the usual glass flare tube 2 sealed at the press portion 3 thereof about a pair of inleads 4 which are formed into clamps 5 engaging the ends of the finished electrode 6. As'illustrated, the greater part of the-electrode is impregnated with electron emission material which completely bridges the turns of the coiled coil filament except for some uncoated turns next to the clamps. The electron-emission material may consist of a mixture of barium strontium and calcium oxides as is well known.

In forming the electrode coil, one may start with a permanent primary tungsten wire 8 along which is laid a temporary mandrel or filler wire 9 of molybdenum as shown in FIG. 2. The wires 8 and 9 together form the composite primary mandrel around which a relatively fine tungsten wire-10 is wrapped to form the overwind. The wires illustrated in FIG. 2 correspond to the sizes used in a 40-watt-fluorescent lamp (F40T12) wherein the overwind 10 is 1 mil tungsten, the primary mandrel 8 is 2.3 mil tungsten, and the temporary mandrel or filler 9 is of 5 mil molybdenum. The overwind turns are actually egg-shaped and of course they are oversized relative to primary mandrel wire 8 and fit quite loosely on it after the filler wire 9 has been removed.

In the next stage of coiling which is generally referred to as the secondary coiling, the use of a single secondary mandrel wire which would cause the secondary turns to be truly circular or helical is avoided. Instead two secondary mandrel wires 11 and 12 consisting of molybdenum wire are laid side by side to form a noncircular secondary mandrel. Around this secondary mandrel the composite result of the primary coiling consisting of the composite mandrel 8,9 with the overwind 10 around it is wound as illustrated in FIGS. 3 and 4. Thereafter the congeries of coils may be set by heat treatment, suitably to about l,500 C in hydrogen. The filler materials or temporary mandrels consisting of primary molybdenum filler 9 and secondary molybdenum mandrels l l and 12 may be dissolved out with the usual molybdenum solvent consisting of a mixture of nitric and sulphuric acids. This leaves the coiled coil 6' illustrated in FIG. 5 and consisting of the single coiled primary tungsten mandrel wire 8 and the coiled coil overwind consisting of fine tungsten wire 10 looping around the secondary turns. Coiled-coil 6 shown in FIG. 5 may then be mounted on a stem as in FIG. 1 and thereafter coated or charged with activating material such as alkaline earth oxides.

Prior to our invention, the ends of the coiled coil had to be crimped, flattened or welded prior to dissolving out the molybdenum mandrels or filler wires. If that was not done, the overwind wire tended to unscrew from the primary tungsten mandrel wire causing tangling and excess shrinkage. Also the flattening or crimping often fractured some of the overwind turns which would fall off and expose the primary mandrel wire, again causing tangling and shrinkage. The present invention eliminates the tendency for the overwind to unscrew after it has been heat set. When the two secondary mandrel wires 11 and 12 are equal in diameter, the secondary turns of the composite are generally oval-shaped. If the two secondary mandrel wires are unequal in diameter, the secondary turns of the composite are egg-shaped. In either case the tendency to unscrew is eliminated and the loose-fitting overwind is retained in place on the non-circular secondary turns.

The composite structure of FIG. 3 consisting of the coiled coil with temporary mandrels or fillers still in place may be coiled a third time around a removable mandrel to form a triple-coiled cathode. The turns thereof will lie against the removable final or tertiary mandrel along a longer side with all turns essentially in the same curving plane thus offering more uniformity in coiling and more consistent cathode performance in the lamp. The finished stem and triple-wound cathode will have essentially the same appearance as in FIGS. 3 and 3a of U.S. Pat. No. 2,774,918 Lemmers, Electric Discharge Device, issued Dec. 18, 1956, which figures are incorporated herein by reference, but the tendency for the overwind to unscrew will be eliminated.

The electrode construction in accordance with our invention has resulted in reduced shrinkage both during coiling and during subsequent lamp making and the quality of the product has been improved.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. A filamentary structure for low pressure discharge lamps comprising a loose fitting overwind of fine refractory wire coiled in oversized primary turns around a permanent primary mandrel wire which in turn is coiled into non-circular secondary turns to form a double-coiled electrode, whereby to prevent the overwind from unscrewing from the primary mandrel wire.

2. A filamentary structure as in claim 1 wherein both wires are of tungsten.

3. A filamentary structure as in claim 1 wherein the secondary turns are oval-shaped.

4. A filamentary structure as in claim 1 wherein both the primary turns and secondary turns are oval-shaped.

5. A filamentary structure as in claim 1 wherein the secondary turns are egg-shaped.

6. A filamentary structure as in claim 1 wherein the primary turns are non-circular and the secondary turns are egg-shaped.

* t i l 

1. A filamentary structure for low pressure discharge lamps comprising a loose fitting overwind of fine refractory wire coiled in oversized primary turns around a permanent primary mandrel wire which in turn is coiled into non-circular secondary turns to form a double-coiled electrode, whereby to prevent the overwind from unscrewing from the primary mandrel wire.
 2. A filamentary structure as in claim 1 wherein both wires are of tungsten.
 3. A filamentary structure as in claim 1 wherein the secondary turns are oval-shaped.
 4. A filamentary structure as in claim 1 wherein both the primary turns and secondary turns are oval-shaped.
 5. A filamentary structure as in claim 1 wherein the secondary turns are egg-shaped.
 6. A filamentary structure as in claim 1 wherein the primary turns are non-circular and the secondary turns are egg-shaped. 